This invention relates to electrically controlled oscillating circuits and more particularly to current or voltage controlled ring oscillators with programmable output frequencies.
Current controlled oscillators (ICO) or voltage controlled oscillators (VCO) are frequently used with phase-locked loop circuits. Typical applications for a phase-locked loop circuit include frequency synthesis or clock generation. When VCOs and ICOs are used in high frequency operations (greater than 100 Mhz), with multi-phased outputs, the oscillator of choice is a ring oscillator.
Ring oscillators have a chain of inverters connected together. The output of one of the inverters is connected in series to the input of another inverter. The output of the last inverter in the chain is connected to the input of the first inverter. Each of the inverters' outputs are tapped to provide signals with a common frequency and different phases. The output frequency is changed by varying the current supply to the oscillator.
The center frequency from oscillator to oscillator typically varies on the order of from 1.5:1 to 2:1 for a given current supply due to inherent process, temperature, and supply voltage variations of the oscillator. Often it is desired to generate output signals over several octave frequency ranges (e.g. 30-125 Mhz) with a ring oscillator while maintaining the range of the current required to change variations in the oscillator as small as possible. A wide output frequency range usually requires that multiple overlapping frequency ranges be used. The restriction on the control current swings is imposed by the voltage swing limitation of a charge pump which supplies current to the oscillator.
Other prior art circuits have used oscillators with programmable numbers of inverters or a single timing stage with programmable timing capacitors. It is not believed that either of these technologies can be used with systems requiring a fixed number of multi-phase outputs with well controlled relative phase shifts over a wide output frequency range.